1. Field of the Invention
This invention relates primarily to a scroll compressor used in refrigerative air conditioning.
2. Description of the Related Art
The compressor unit in motor driven compressors used in refrigerative air conditioning may be a reciprocating, rotary, or scroll compressor. Of these, the scroll compressor is widely used in practice to obtain its advantages of high efficiency, low noise, and low vibration.
The basic structure, as is well known, comprises a fixed scroll wherein a spiral wrap rises from a base plate, and an orbiting scroll wherein a spiral wrap of more or less the same shape rises from a base plate, both scrolls being meshed together and forming a plurality of compression chambers therebetween. When the orbiting scroll is made to move orbitally about the fixed scroll without itself rotating, the compression chambers suck in coolant from open parts at the periphery of the base plates, and then close as they move toward the center of the base plates, so that their volumes gradually diminish, compressing the coolant and finally discharging it.
The compression ratio of refrigerative air conditioning is normally set between 1.8 and 2.7 or so. However, in the face of increasing demand for smaller sizes and greater operating mode diversification in recent years, there is a trend toward designing the device to have a lower compression ratio in the interest of efficiency.
Japanese Published Unexamined Patent Application Tokkaihei 5-202871 discloses a technology for increasing the coolant discharge volume and preventing a decline in volume efficiency with inducted coolant being heated by the heat of compression. FIG. 13 shows the construction of such device, in which the inside wall surface 100b at the spiral end of a wrap 100a of a fixed scroll 100 is extended near to the spiral end of a wrap 102a of an orbiting scroll 102. The outer wall surface 102b of the orbiting scroll 102 from the part opposing the spiral end of the wrap 100a of the fixed scroll 100 is continuously displaced inwards toward the spiral end of the orbiting scroll 102. The inside wall surface 100b in the extended portion of the fixed scroll 100 is formed by the envelope associated with the orbital motion of the outside wall surface 102b at the spiral end of the orbiting scroll 102.
In this device, an additional outside operative space 104 is formed between the inside wall surface 100b of the extended portion of the wrap 100a of the fixed scroll 100 and the outside wall surface 102b of the orbiting scroll 102, and the intake space of the compression chamber 106 increases by this added amount, whereupon the coolant discharge volume is increased. Moreover, since the coolant is inducted into the compression chamber 106 directly from an inlet port 108, it does not come into contact with walls through which compression heat is conducted, and is not heated, wherefore volume efficiency does not decline due to thermal expansion.
However, with the increasing diversification of operating modes, scroll compressors must now operate with variable speeds under inverter control. When variable speed operation is implemented, a wide range of operating conditions can be realized with the same scroll compressor, from apparently high-power operation at high capacity to low-power operation at low capacity.
When such variable speed operation is conducted, however, the volume efficiency is not observed to improve markedly even at high speed operation, as indicated in FIG. 3, and no improved efficiency can be hoped for. Even with the prior art, as graphed in FIG. 3, all of the envelope portion noted above operates as a compression chamber, and coolant discharge volume increases, but, as plotted in FIG. 4C, the power required for compression increases irrespective of the operating speed, and efficiency cannot be greatly improved. In particular, during low-power operations to achieve a high energy saving effect, it becomes necessary to lower the operating speed even further, whereupon efficiency declines markedly due to increased coolant leakage.